1. Field of the Invention
The present invention relates to a polishing slurry to polish high-hardness materials, particularly to a polishing technology capable of speedily and smoothly polishing high-hardness materials such as silicon carbide and gallium nitride.
2. Description of the Related Art
Among semiconductor devices, in power semiconductor elements, so-called power devices, use of silicon carbide, gallium nitride, diamond or the like as substrates in place of silicon, which has been conventionally used, is proposed for the purpose of higher breakdown voltage and larger current. These substrates composed of silicon carbide or the like, since having a larger band gap than silicon substrates, can resultantly withstand higher voltages. The high-breakdown voltage property of the substrates composed of silicon carbide, gallium nitride or the like is deemed to be originated from the fact that the atomic arrangement of atoms constituting the silicon carbide or the like is denser than silicon.
By contrast, due to a particularly high hardness, substrates composed of silicon carbide, gallium nitride or the like have a problem of being hardly capable of being polished with polishing materials, which have been used conventionally. Silicon carbide and the like, since having a dense atomic arrangement as described above, have particularly a high hardness, and are high-hardness materials whose hardnesses in terms of Mohs hardness are about 9 for silicon carbide and gallium nitride and 10 for diamond. In order to polish these high-hardness materials, materials having a high hardness, such as diamond and aluminum oxide, have been used as an abrasive particle. However, if polishing is carried out by use of diamond or the like, only mechanical polishing progresses, which thereby causes defects and distortions to be easily generated in substrates, thus posing a risk of lacking the reliability of devices. Such a tendency becomes more emphatic, the higher the hardness of substrates.
In order to cope with the above problem, there are proposed polishing materials, as polishing materials for high-hardness materials such as silicon carbide and gallium nitride, in which an oxidative solution of hydrogen peroxide, ozone, permanganic acid or the like is added to an abrasive particle of silicon oxide, aluminum oxide or the like (Patent Literatures 1 to 3). There are proposed an polishing material whose polishing power is enhanced by use of chromium oxide together with a powdery or liquid oxidizing agent (Patent Literature 4), and also an polishing material whose polishing power is enhanced without addition of an oxidative solution by use of an abrasive particle having a specific shape (Patent Literature 5). There are also proposed applicable materials as abrasive particles and oxidizing agents (Patent Literatures 6, 7 and 8). For polishing treatment of high-hardness materials, there is further proposed an polishing material in which manganese dioxide is disposed as an outermost surface layer of an abrasive particle of silicon oxide or silicon carbide by coating or the like (Patent Literature 9).